This application relates to an auxiliary power unit (“APU”) which is put through a cool down cycle at shutdown, but only in an extreme temperature situation.
APUs are known, and are typically provided in an aircraft, and started prior to actuation of the main aircraft gas turbine engines. The APU provides electric power through a generator for use on the aircraft prior to the startup of the main gas turbine engines. In addition, the APU may supply air for use in the aircraft cabin, and to start the aircraft engines.
Once the main aircraft engines have started, then the APU may be shut down. The APUs typically are under a heavy load just prior to shutdown, in that they are supplying a good deal of electricity, plus air to start the main gas turbine engines. Thus, they can be quite hot at shutdown.
Aircraft tail cones typically house the APU. The aircraft tail cones have traditionally been made of a metal, however, more recently they have been formed of various fiber composite materials. As an example, carbon fiber materials are being utilized.
Such fiber materials cannot withstand the high temperatures that the prior metal tail cones could withstand. As an example, an aluminum tail cone can withstand 450° Fahrenheit (232° Celsius) while the composite tail cones may only withstand 200° Fahrenheit (93° Celsius).